This issue was fixed by H.J. a few days ago, but it reappeared today:

 make[4]: Entering directory `/var/tmp/gcc_build_dir/x86_64-pc-linux-gnu/libsanitizer/tsan'
test -z "/usr/lib/gcc/x86_64-pc-linux-gnu/" || /bin/mkdir -p "/usr/lib/gcc/x86_64-pc-linux-gnu/"
 /bin/sh ../libtool   --mode=install /usr/bin/install -c   libtsan.la '/usr/lib/gcc/x86_64-pc-linux-gnu/'
libtool: install: error: cannot install `libtsan.la' to a directory not ending in /usr/lib/gcc/x86_64-pc-linux-gnu/
make[4]: *** [install-toolexeclibLTLIBRARIES] Error 1

Configured with:
~/gcc/configure --prefix=/usr --bindir=/usr/x86_64-pc-linux-gnu/gcc-bin/4.8.0 --includedir=/usr/lib/gcc/x86_64-pc-linux-gnu/4.8.0/include --datadir=/usr/share/gcc-data/x86_64-pc-linux-gnu/4.8.0 --mandir=/usr/share/gcc-data/x86_64-pc-linux-gnu/4.8.0/man --infodir=/usr/share/gcc-data/x86_64-pc-linux-gnu/4.8.0/info --with-gxx-include-dir=/usr/lib/gcc/x86_64-pc-linux-gnu/4.8.0/include/g++-v4 --host=x86_64-pc-linux-gnu --build=x86_64-pc-linux-gnu --disable-altivec --disable-fixed-point --without-ppl --without-cloog --enable-lto --enable-nls --without-included-gettext --with-system-zlib --disable-werror --enable-initfini-array --with-gold --enable-secureplt --disable-multilib --enable-libmudflap --disable-libssp --disable-libgomp --enable-cld --with-python-dir=/share/gcc-data/x86_64-pc-linux-gnu/4.8.0/python --enable-checking=release --disable-libgcj --enable-languages=c,c++ --enable-shared --enable-threads=posix --enable-__cxa_atexit --enable-clocale=gnu --with-build-config=slim-lto-bootstrap --with-boot-ldflags="-Wl,-O1,--hash-style=gnu,--as-needed,--gc-sections,--icf=safe,--icf-iterations=3" --enable-version-specific-runtime-libs --disable-libstdcxx-pch --enable-libstdcxx-time=yes